My guess is it can't be someone you just sent down within a certain number of days. I forget the exact rule but there's something like that - also, they seem to be hoping it's a day to day thing and they won't be needing him to actually play.

Reading the injury report, it sounds like Gustavsson could be back as soon as next game. McCollum played yesterday with GR and Mrazek is slated to play tonight, so it makes sense for a number of reasons since it's only bench duty.